Impact of Slip Cycles on the Operation Modes and Efficiency of Molecular Motors

Kinesin is a motor molecule that moves processively on microtubule tracks and is involved in active intracellular transport processes. For small loads, it is powered by the hydrolysis of one ATP molecule per step. Here we extent our previously introduced network theory in order to study the possibility of two different mechanical stepping transitions and the general behavior of the motor's efficiency. Our theory shows explicitly how chemical and mechanical slip cycles emerge that weaken the coupling between ATP hydrolysis and mechanical stepping. Near chemomechanical equilibrium, the motor efficiency eta may vary between eta=1 for tight coupling and eta=0 for loose coupling, depending on the relevance of the slip cycles. Far from chemomechanical equilibrium, on the other hand, the motor efficiency is found to decay as 1/Delta mu with increasing Delta mu irrespective of the presence of slip cycles, where Delta mu represents the reaction free enthalpy or chemical potential difference per ATP hydrolysis.